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The emerging field of high-temperature solar thermochemistry holds tremendous promise to disrupt the conventional paths of producing fuels, energy-intensive commodity materials and generating dispatchable power. Direct solar irradiation is concentrated by mirrors, captured by a receiver, converted into high-temperature process heat, often exceeding $\mathrm{1000K}$, and used to drive highly-endothermic processes for production of chemical fuels and materials including synthesis gas, carbon black, ammonia, lime, and metals \cite{bader2016solar}\cite{Fletcher_2001}\cite{Steinfeld_2003}\cite{Kodama_2003}. Production of the components of synthesis gas, $\mathrm{H_2}$ and $\mathrm{CO}$, can be achieved in a single thermal dissociation step (thermolysis) followed by a high-temperature gas separation. However, the excessively high temperatures required for such chemistry pose serious limitations for practical implementation. For example, the thermolysis of $\mathrm{H_2O}$ and $\mathrm{CO_2}$ require temperatures exceeding $\mathrm{2500K}$ in order to obtain significant $\mathrm{H_2}$ and $\mathrm{CO}$ concentrations, respectively \cite{bader2016solar}. Alternatively, thermochemical cycles can be employed, in which active materials undergo cyclic reduction and oxidation (redox) reactions, with the net effect of splitting $\mathrm{H_2O}$ and $\mathrm{CO_2}$. In addition to lowering process temperatures, the added process steps can be leveraged to eliminate energetically expensive gas-phase separation requirements.   High-temperature solar thermochemical reactors are designed for maximum solar-to-chemical energy conversion efficiency, to minimise the required size of the optical concentrators, and thus to improve the economic viability of the process \cite{Palumbo_2004}. The rapid and efficient transfer of energy from a concentrated solar source to the redox material---i.e. the transfer of energy from photons to chemical bonds---is therefore a core critical research aspect towards achieving an high-output and efficient reactor design. To this end, we study here the performance of a class of porous particles of cerium dioxide redox materials under highly-concentrated, direct solar irradiation. We believe these materials to hold promise for efficient energy transfer for a number of reasons we outline here. More specifically, we will  answer the questions: Why particles? Why porous? Why ceria? "Why particles?", "Why porous?", and "Why ceria?" while outlining the relevant literature in the process.  Unlike bulk materials, particles can have an absorption efficiency (potentially much) greater than unity.   Note that this may seem impossible given the fact absorption efficiency must be equal to emissivity by Kirchoff's law and that a blackbody---the perfect emitter---cannot have an emissivity greater than one. However, the idea of a perfect blackbody is restricted to surfaces with low curvature. Both Planck and Kirchoff were clear on this when deriving their famous laws \cite{bohren1998absorption}. Thus, for a material to absorb very well relative to its size---the trait needed to achieve high energies within a particle with relatively little input---we need to look at materials with features on the scale of the wavelength of light. Perhaps the simplest conceivable geometry is the sphere as we consider here.   Ceria has received significant attention as a redox material because of its relatively fast reaction rates and ability to remain in a solid state throughout the redox process which relieves previous problems associated with ferrite-based oxides and volatile metal oxides such as $\mathrm{ZnO}$ and $\mathrm{SnO_2}$ \cite{Chueh_2010a}. The demonstrated ability of ceria to be used as a redox active material and the availability of relevant data make it a very strong candidate to be optimized for high-efficiency thermochemical cycling. The spectral optical properties of ceria are well-known \cite{Patsalas_2002} at room temperature in its stoichiometric form and the thermodynamic properties of non-stoichometric ceria are available allowing detailed modeling in pursuit of this goal.  Porous materials offer numerous benefits over solid materials for such radiation-driven thermochemistry applications. The high surface area and small volumes we consider allow for rapid transport of oxygen release and uptake in the solid ceria matrix. Porous particles also have lower density leading to lower necessary carrier velocities when fluidized. A degree of radiative property tunability was shown to be attainable by varying the internal pore structure  as well as the overall particle size. Larger pores were shown to lead to broadening and decay of the absorption  efficiency factor peak, redshifting and decay of the scattering efficiency factor peak, and a trend away from  isotropic scattering. Increases in particle size leads to similar effects \cite{Randrianalisoa_2014}.   Since ceria naturally absorbs well only in the ultraviolet region, any redshifting of  of absorption peaks should lead to significant improvements in solar-weighted absorption. A scattering peak near  the peak of the solar spectrum is also desirable to reduce direct irradiation of the back of the reactor   by redistributing radiation within the cloud allowing for more chance of absorption.   In this paper, we highlight the dramatic effect such tunability can have on the macroscopic behavior of a cloud   of such particles undergoing thermal reduction.  \begin{itemize}  \item  transport of oxygen into and out of solid ceria is easier  \item  lower density so easier to keep in suspension... allows flexibility in choosing flow rate to adjust for changes in solar flux... also tradeoff with convective losses  \item  geometry may lead to high absorption or radiative energy \textit{per volume} of ceria  \item  increased surface area leads to increased reaction release and uptake  \item  A degree of radiative property tunability was shown to be attainable by varying the internal pore structure  as well as the overall particle size. Larger pores were shown to lead to broadening and decay of the absorption  efficiency factor peak, redshifting and decay of the scattering efficiency factor peak, and a trend away from  isotropic scattering. Increases in particle size leads to similar effects \cite{Randrianalisoa_2014}.   Since ceria naturally absorbs well only in the ultraviolet region, any redshifting of  of absorption peaks should lead to significant improvements in solar-weighted absorption. A scattering peak near  the peak of the solar spectrum is also desirable to reduce direct irradiation of the back of the reactor   by redistributing radiation within the cloud allowing for more chance of absorption.   In this paper, we highlight the dramatic effect such tunability can have on the macroscopic behavior of a cloud   of such particles undergoing thermal reduction.  \end{itemize}  Literature to cite including an egregious amount of self-citations as per Lipi\'{n}ski standard: